centerVertically.js—a simple extension to the jQuery library that allows you to vertically center most anything.

centerVertically.js

/**
 * centerVertically() takes a given DOM element and centers 
 * it vertically within the parent element. If the height
 * of the browser is less than this element's height, then
 * the original margin relative to top is restored until
 * the window gets larger again.
 *
 * Note that this function extends the jQuery library and
 * therefore requires it before this function can be
 * defined.
 *
 * E.g. usage:
 * $('.wrap').centerVertically();
 */
(function( $ ){
	$.fn.centerVertically = function() {
		var elem = $(this);
		var height = $(elem).outerHeight();
		var marginTop = $(elem).css('marginTop');

		var adjustMargin = function() {
			if ( $(window).height() > height )
			{
				$(elem).css('margin-top', ( $(window).height() - height ) / 2 + 'px');
			}
			else
			{
				$(elem).css('margin-top', marginTop);		
			}
		}
		adjustMargin();
		$(window).resize( adjustMargin );
	};
})( jQuery );